Thin film transistor with a poly-crystalline semiconductor film (poly-crystalline TFT) is superior to TFT with an amorphous semiconductor film in its mobility by double digits or more. The poly-crystalline TFT has an advantage that a pixel portion and its peripheral driver circuit in a semiconductor display device can be integrated on the same substrate.
The poly-crystalline semiconductor film can be formed over an inexpensive glass substrate when a laser annealing method is employed. However, the energy of the laser light output from the oscillator fluctuates by at least a few percentage points due to the various reasons. This fluctuation prevents homogeneous crystallization of the semiconductor film. When the crystallinity of the poly-crystalline semiconductor film varies due to the inhomogeneous crystallization, the characteristic of TFT using the poly-crystalline semiconductor film as its active layer, such as on-current or mobility also varies.
For example, in the case of an active matrix light-emitting device in which a light-emitting element and TFT for controlling current supply to the light-emitting element are provided in each pixel, when the on-current of TFT varies, the luminance of the light-emitting element also varies accordingly.
FIG. 18(C) shows the energy density in every shot of pulsed laser light. FIG. 18(B) shows the drain current of TFT for supplying the current to the light-emitting element in the pixel corresponding to every shot. FIG. 18(A) is a photograph of the pixel portion emitting light in fact. It is noted that the photograph of the pixel portion in FIG. 18(A) is extended to the lateral direction in accordance with the number of the steps in the scanning direction and with the number of shots in order to make it easier to understand the relation with the graphs shown in FIG. 18(B) and FIG. 18(C).
The luminance of the light-emitting element is in proportion to the current intensity in the light-emitting element and the current intensity in the light-emitting element is almost equal to the drain current intensity of the TFT supplying the current to the light-emitting element. When FIGS. 18(B) and 18(C) are compared, it is understood that the drain current of TFT corresponding to the shot having high energy density is also high. And when FIGS. 18(A) and 18(B) are compared, it is understood that when the drain current is high, the luminance of the corresponding part in the pixel portion is also high.
Therefore, FIG. 18 shows that the energy fluctuation of the laser light corresponds to stripes appeared as light and shade of luminance in the pixel portion of the light-emitting device.
Moreover, when the semiconductor film is crystallized by irradiating the laser light in the atmosphere, the surface of the semiconductor film becomes somewhat rough. The higher the energy intensity of the laser light is, the rougher the surface of this semiconductor film becomes. The light is scattered to give more brightness in the region whose surface is rougher. Therefore, sometimes the striped light and shade is visible at an interval of several mm due to the energy fluctuation.
It is noted that the state of the surface of the semiconductor film is closely related to the oxygen in the atmosphere at the laser light irradiation according to the patent document 1 shown below.
[Patent Document 1]
Published patent application No. 2000-138180 (P. 3–P. 4)
The patent document 1 describes that the more oxygen the atmosphere contains, the rougher the surface of the semiconductor film crystallized by irradiating the laser light becomes. Moreover, the patent document 1 also describes to spray the semiconductor film with Ar when the laser light is irradiated.
When the surface of the semiconductor film becomes rough, interface state density at the interface between the semiconductor film and the gate insulating film formed so as to contact the semiconductor film becomes high and the threshold voltage shifts to the normally-off side. Therefore, when the state of the surface of the semiconductor film becomes uneven due to the energy fluctuation of the laser light, the interface state density varies at the interface between the semiconductor film and the gate insulating film formed afterward, and as a result the threshold of TFT varies.